The present invention relates generally to measurement of a fluid flow. More particularly the invention relates to a flow sensor according to the preamble of claim 1, an ultra-filtration measuring unit according to the preamble of claim 8 and a method for measuring a flow rate component according to the preamble of claim 10.
There are many technical areas in which an accurate fluid flow measurement is required. Numerous representative applications can, for instance be found in the field of medical technology. However, some kind of flow measurement is normally required in most instances where a fluid/liquid is to be transported from one point to another. Fluid flow measurements may also be used to determine the velocity of a craft traveling in water or a similar liquid. Naturally, mechanical flow sensors may here be employed. Nevertheless, when a high accuracy is required, electromagnetic flow measurement sensors are generally preferable. This type of sensors are based on a well-known technique where a magnetic field is applied to interact with electrically charged elements, such as ions, in the flowing fluid to produce a resulting electric field. Thus, according to Faraday's law, the magnitude of the induced electric field constitutes a measure of the flow rate of the fluid.
The applicant develops and manufactures medical equipment, for instance renal products in the form of dialysis apparatuses, which utilize such an electromagnetic flow measurement technique. Commonly, flow sensors with platinum electrodes have been employed to deliver accurate and reliable values of the registered fluid flows. Platinum, and platinum black in particular, namely accomplishes a good electrical contact between the electrodes and the dialysis liquid, so that the electrode dimensions can be held comparatively small, and consequently give rise to a linear magnetic field pattern in the fluid conduit cross section.
In recent years, a so-called glucose-charging practice has been introduced wherein glucose is added to the dialysis liquid in order to better imitate the composition of the patient's own blood, and thereby i.a. avoid certain metabolic risks for the patient. However, glucose also produces undesired effects due to its electrochemical activity with platinum. During operation of a dialysis apparatus with a glucose-charged dialysis liquid, a catalytic reaction causes glucose to be oxidized on the flow sensors' platinum surfaces. This decreases the reliability of the flow measurements. Namely, the oxidation may result in a varying DC-level (DC=Direct Current) at the sensor electrodes, which in turn renders it difficult to determine the contribution to the registered electric field caused by the electromagnetic interaction between the applied magnetic field and the charged elements in the flowing liquid (i.e. here ions in the dialysis liquid).
The U.S. Patent Application No. 2002/0050175 describes a magnetic flow sensor and method, wherein an undesired drift of the electrode voltages is compensated for, either by interconnecting the sensing electrodes or by connecting them to a common potential, such as ground. Also in this case, a voltage indicative of the flow rate is measured by means of at least two electrodes. A high-impedance voltage-measurement circuit is used to register a voltage between the electrodes. Thus, during the measurement, the electrodes are in an open circuit state, and may therefore be electrically influenced by electrode polarization and other measurement error-inducing factors that develop relatively slowly. In order to avoid such effects, the electrodes are in a closed circuit state for most of the time and placed in an open circuit state only during a relatively brief measurement interval portion of the operating cycle.
Although this strategy may solve the drifting problem, it results in a limited maximum sampling frequency, and consequently also an accuracy constraint with respect to the detection of rapid changes in the flow rate. Moreover, the procedure requires a considerable amount of high-speed switching, and is therefore both expensive to implement and relatively prone to malfunction.